Shock absorbers are widely used in the suspension systems of recreational vehicles such as snowmobiles and all terrain vehicles. Shock absorbers dampen shocks experienced when the recreational vehicle travels over rough terrain. Shock absorbers are typically mounted between a vehicle component that moves in relation to the chassis and the chassis itself. Shock absorbers are often used in combination with a spring assembly which may or may not be integrated with the shock absorber. In a snowmobile, shock absorbers are typically positioned between the chassis and the slide frame around which an endless track rotates to propel the vehicle. The shock absorber(s) allow the slide frame to compress towards the chassis at a controlled rate. In the case of an all terrain vehicle, the shock absorbers are typically positioned between a wheel assembly and the chassis. The shock absorber(s) allow the wheel assembly to compress towards the chassis at a controlled rate.
Shock absorbers typically have a shock body having a cylindrical wall sealed between first and second end caps creating a chamber in which a fluid is contained. The interior of the shock body is separated into two sections by a piston, which moves within the fluid. Shock absorbers typically include a shock rod having a first end attached to the piston, defining a shock rod/piston assembly, and a second end attached to the vehicle frame or chassis. Normally the shock rod is attached to the vehicle chassis through a rod eye. The first end cap, which is typically at the bottom of the shock body includes a mounting structure suitable for coupling to a vehicle component that moves in relation to the chassis. In the case of a snowmobile, the end cap is coupled to the slide frame. In the case of an all terrain vehicle, the end cap is coupled to a frame component. The shock rod extends through the second end cap of the shock body which is named the “rod-eye end cap.” The rod-eye end cap is typically disposed at the top of the shock body.
For the piston to move within the shock body, the fluid within the fluid-filled chamber of the shock body must travel through the piston. Therefore, passages are formed through the piston to control the fluid flow between each section of the shock body. The passages are typically aligned with the longitudinal axis of the piston. The openings of some of these passages may be covered with leaf valves while the remainder of the openings may be uncovered to thus serve as by-pass passages. The only restriction in the by-pass passages is the viscosity of the fluid itself and the diameter of the passages.
The shock rod/piston assembly and the shock body (which includes the cylindrical wall and both of the end caps) move in relation to one another upon the application of forces to the shock absorber. The relative movement between the shock rod/piston assembly and the shock body results in the movement of the piston through the fluid, which provides the hydraulic damping for the shock absorber. Therefore, the shock forces that are applied to the vehicle component to which the shock absorber is coupled are at least partially absorbed by the shock absorber. Accordingly, the shock forces that are applied to the vehicle frame or chassis are dampened by the shock absorber.
The movement of the shock rod/piston assembly within the fluid-filled chamber of the shock body occurs in two stages, a compression stage followed by a rebound stage, both of which are described in greater detail below.
As the vehicle runs over rough terrain, shock forces are applied to the vehicle component to which the shock absorber is mounted. These shock forces cause the vehicle component to move from a steady state position to one where the vehicle component has compressed relative to the chassis. Since the shock absorber is disposed between the vehicle component and the chassis, as the components move toward one another, the shock absorber compresses. This is called the compression stage of the shock absorber. As the shock absorber compresses, the shock rod/piston assembly moves inwardly relative to the shock body, within the fluid-filled chamber of the shock body. As a result, the piston moves within the fluid-filled chamber of the shock body toward the first end cap. During this compression stage, the shock absorber slows or dampens the rate at which the vehicle component compresses toward the chassis.
The rebound stage follows the compression stage. The rebound stage results from the resilient expansion of the spring associated with the shock absorber, which pushes the vehicle component away from the vehicle chassis to the original steady state position. The force exerted by the spring is usually quite low by comparison with the compressive force, because, in the rebound stage, the force of the spring only needs to be high enough to overcome the combined weight of the vehicle and the rider. This spring force causes the shock absorber to extend, resulting in the shock rod/piston assembly extending outwardly relative to the shock body. The piston moves within the fluid-filled chamber away from the first end cap toward the second or “rod eye” end cap. As was the case during the compression stage, the shock absorber slows or dampens the rate at which the vehicle component may move relative to the chassis during the rebound stage.
During the compression stage, the shock rod/piston assembly moves inwardly within the shock body toward the shock body first end cap. Accordingly, the shock rod displaces a volume of fluid within the shock body that is equal to the volume of the shock rod that has extended into the shock body. To accommodate this displacement of fluid, a reservoir is typically used in association with the shock absorber. As fluid within the shock body fluid-filled chamber is displaced by the shock rod, the volume of fluid in the reservoir increases a corresponding amount. During the rebound stage, the shock rod/piston assembly moves outwardly from the shock body away from the shock body first end cap. Accordingly, the fluid within the reservoir that was displaced by the shock rod during the compression stage re-enters the shock body.
In some shock absorbers, a valve separates the shock body fluid-filled chamber from the reservoir. The valve controls the rate at which fluid may pass from the shock body fluid-filled chamber to the reservoir, and/or the rate at which fluid may pass from the reservoir back to the shock body fluid-filled chamber. In some instances, the valve may restrict the movement of fluid to a single direction between the shock body fluid-filled chamber and the reservoir. In this situation, the shock absorber must include a structure through which the fluid may move in the opposite direction between the shock body fluid-filled chamber and the reservoir. A second valve may be used for this purpose. A valve that controls the flow of fluid from the shock body fluid-filled chamber to the reservoir is usually called a compression valve. A valve that controls the flow of fluid from the reservoir to the shock body fluid-filled chamber is usually referred to as a rebound valve.
In some prior art shock absorber designs, both of these valves are adjustable. By controlling the rate at which fluid may pass from the shock body fluid-filled chamber to the reservoir, or vice versa, the valves control the rate at which the shock rod moves in relation to the shock body. As the fluid in the shock body fluid-filled chamber is incompressible, the shock rod can enter the shock body only at substantially the same rate at which the volume of the shock rod displaces fluid from the shock body fluid-filled chamber into the reservoir. Obviously, if the rate at which the fluid may be displaced is changed, then the rate at which the shock rod may move in relation to the shock body will be changed to a corresponding amount.
Many prior art shock absorbers fail to provide sufficient adjustment to the rate at which fluid is permitted to pass through the compression and rebound valves. Such inadequacies may result from the incorporation of valves with too small an adjustment range. Other prior art shock absorbers provide no adjustment. Still others provide adequate adjustment by providing an adjustment knob or selector, but the adjustment knob or selector is positioned such that it is largely inaccessible (or not conveniently accessible) to the vehicle operator.
When provided, compression and rebound valves are typically disposed on the shock absorber at a location adjacent to the shock body first end cap. Accordingly, an adjustment knob or selector typically is disposed at this position on the shock absorber. The first end cap usually includes a mounting structure such as an eye that is used to attach the shock absorber to a movable vehicle component. Accordingly, the adjuster knob or selector often is placed in close proximity to the movable vehicle component in a position the operator may find difficult to access. In these shock absorber designs, the adjustment feature is of little use because the user will likely disregard a feature that is not convenient to use.
Some shock absorber designs use a reservoir that is open to the atmosphere through a vent hole. In these shock absorber designs, if the pressure of the fluid that enters the reservoir is high and cannot be modulated, the fluid may be undesirably discharged from the shock absorber through the vent hole. This undesirable discharge is known as leakage and results in diminished shock absorber performance. It may also have the undesirable effect of covering vehicle parts with shock absorber fluid.
A need, therefor, has developed for a shock absorber that maximizes the ease with which an adjustment may be made to the rate at which the shock rod may move in relation to the shock body. A need has also developed for a shock absorber that maximizes the extent to which an adjustment may be made to the rate at which the shock rod may move in relation to the shock body. A need has also developed for a shock absorber that includes a vent to the atmosphere, but which minimizes the possibility that shock fluid may be undesirably discharged from the shock absorber to the ambient environment. The prior art does not address these aforementioned needs.